1. Field of the Invention
The present invention relates to an image reading apparatus used for reading an image of a document or other desired images.
2. Description of the Related Art
A typical image reading apparatus includes a plurality of sensor IC chips which are arranged in a row and each of which incorporates a plurality of light receiving elements as photoelectric conversion elements. Upon receiving light reflected by a document illuminated by a light source, the plurality of light receiving elements output image signals of a level corresponding to the received amount of light as serial analog signals.
With the image reading apparatus having such a structure, reading of an image for one line by successively driving the plurality of sensor IC chips may take a relatively long time when the number of pixels to be read is relatively large. Particularly, for reading a color image of a document, three kind of light sources respectively emitting red light, blue light and green light are provided, and an image for one line need be read three times by successively driving the three kind of light sources, which takes longer time.
In view of the above, JP-A-10-285330 proposes an image reading apparatus in which a plurality of sensor IC chips aligned in a row are divided into a plurality of blocks so that the sensor IC chips are driven on a block-by-block basis. Specifically, in one embodiment, a single D/A converter common to all the blocks is provided, and conversion of analog image signals from the sensor IC chips into digital image signals is performed with respect to each of the blocks with time differences for storage in a memory (time difference D/A conversion). In another embodiment, D/A converters of a number corresponding to the number of blocks are provided, and conversion of analog image signals from the sensor IC chips into digital signals is performed with respect to all the blocks in parallel with each other for storage in a memory (parallel D/A conversion).
When the time difference D/A conversion is utilized, the time required for signal processing becomes long due to the time difference provided among the blocks. Further, the controlling for providing time difference is complicated. On the other hand, the provision of D/A converters of a number corresponding to the number of blocks increases the cost for the image reading apparatus.